TELNET(1) BSD General Commands Manual TELNET(1)
NAME
telnet — user interface to the TELNET protocol
SYNOPSIS
telnet [-8EFKLacdfrx] [-X authtype] [-b hostalias] [-e escapechar] [-k realm] [-l user] [-n tracefile] [host
[port]]
DESCRIPTION
The telnet command is used to communicate with another host using the TELNET protocol. If telnet is invoked
without the host argument, it enters command mode, indicated by its prompt (telnet>). In this mode, it accepts
and executes the commands listed below. If it is invoked with arguments, it performs an open command with those
arguments.
The options are as follows:
-7 Strip 8th bit on input and output. Telnet is 8-bit clean by default but doesn't send the TELNET BINARY
option unless forced.
-8 Specifies an 8-bit data path. This causes an attempt to negotiate the TELNET BINARY option on both
input and output.
-E Stops any character from being recognized as an escape character.
-F If Kerberos V5 authentication is being used, the -F option allows the local credentials to be forwarded
to the remote system, including any credentials that have already been forwarded into the local environ‐
ment.
-K Specifies no automatic login to the remote system.
-L Specifies an 8-bit data path on output. This causes the BINARY option to be negotiated on output.
-X atype
Disables the atype type of authentication.
-a Attempt automatic login. Currently, this sends the user name via the USER variable of the ENVIRON
option if supported by the remote system. The name used is that of the current user as returned by
getlogin(2) if it agrees with the current user ID, otherwise it is the name associated with the user ID.
-b hostalias
Uses bind(2) on the local socket to bind it to an aliased address (see ifconfig(8) and the ``alias''
specifier) or to the address of another interface than the one naturally chosen by connect(2). This can
be useful when connecting to services which use IP addresses for authentication and reconfiguration of
the server is undesirable (or impossible).
-c Disables the reading of the user's .telnetrc file. (See the toggle skiprc command on this man page.)
-d Sets the initial value of the debug toggle to TRUE.
-e escapechar
Sets the initial telnet escape character to escapechar. If escapechar is omitted, then there will be no
escape character.
-f If Kerberos V5 authentication is being used, the -f option allows the local credentials to be forwarded
to the remote system.
-k realm
If Kerberos authentication is being used, the -k option requests that telnet obtain tickets for the
remote host in realm realm instead of the remote host's realm, as determined by krb_realmofhost(3).
-x Turns on encryption of the data stream if possible.
host Indicates the official name, an alias, or the Internet address of a remote host.
port Indicates a port number (address of an application). If a number is not specified, the default telnet
port is used.
When in rlogin mode, a line of the form ~. disconnects from the remote host; ~ is the telnet escape character.
Similarly, the line ~^Z suspends the telnet session. The line ~^] escapes to the normal telnet escape prompt.
Once a connection has been opened, telnet will attempt to enable the TELNET LINEMODE option. If this fails,
telnet will revert to one of two input modes: either ``character at a time'' or ``old line by line'' depending
on what the remote system supports.
When LINEMODE is enabled, character processing is done on the local system, under the control of the remote sys‐
tem. When input editing or character echoing is to be disabled, the remote system will relay that information.
The remote system will also relay changes to any special characters that happen on the remote system, so that
they can take effect on the local system.
In ``character at a time'' mode, most text typed is immediately sent to the remote host for processing.
In ``old line by line'' mode, all text is echoed locally, and (normally) only completed lines are sent to the
remote host. The ``local echo character'' (initially ``^E'') may be used to turn off and on the local echo
(this would mostly be used to enter passwords without the password being echoed).
If the LINEMODE option is enabled, or if the localchars toggle is TRUE (the default for ``old line by line'';
see below), the user's quit, intr, and flush characters are trapped locally, and sent as TELNET protocol
sequences to the remote side. If LINEMODE has ever been enabled, then the user's susp and eof are also sent as
TELNET protocol sequences, and quit is sent as a TELNET ABORT instead of BREAK. There are options (see toggle
autoflush and toggle autosynch below) which cause this action to flush subsequent output to the terminal (until
the remote host acknowledges the TELNET sequence) and flush previous terminal input (in the case of quit and
intr).
While connected to a remote host, telnet command mode may be entered by typing the telnet ``escape character''
(initially ``^]''). When in command mode, the normal terminal editing conventions are available. Note that the
escape character will return to the command mode of the initial invocation of telnet that has the controlling
terminal. Use the send escape command to switch to command mode in subsequent telnet processes on remote hosts.
The following telnet commands are available. Only enough of each command to uniquely identify it need be typed
(this is also true for arguments to the mode, set, toggle, unset, slc, environ, and display commands).
auth argument [...]
The auth command manipulates the information sent through the TELNET AUTHENTICATE option. Valid
arguments for the auth command are as follows:
disable type Disables the specified type of authentication. To obtain a list of available types,
use the auth disable ? command.
enable type Enables the specified type of authentication. To obtain a list of available types, use
the auth enable ? command.
status Lists the current status of the various types of authentication.
close Close a TELNET session and return to command mode.
? command.
enable type [input|output]
Enables the specified type of encryption. If you omit input and output, both input and
output are enabled. To obtain a list of available types, use the encrypt enable ? com‐
mand.
input This is the same as the encrypt start input command.
-input This is the same as the encrypt stop input command.
output This is the same as the encrypt start output command.
-output This is the same as the encrypt stop output command.
start [input|output]
Attempts to start encryption. If you omit input and output, both input and output are
enabled. To obtain a list of available types, use the encrypt enable ? command.
status Lists the current status of encryption.
stop [input|output]
Stops encryption. If you omit input and output, encryption is on both input and out‐
put.
type type Sets the default type of encryption to be used with later encrypt start or encrypt stop
commands.
environ arguments [...]
The environ command is used to manipulate the variables that may be sent through the TELNET ENVIRON
option. The initial set of variables is taken from the users environment, with only the DISPLAY and
PRINTER variables being exported by default. The USER variable is also exported if the -a or -l
options are used.
Valid arguments for the environ command are:
define variable value
Define the variable variable to have a value of value. Any variables defined by this
command are automatically exported. The value may be enclosed in single or double quotes
so that tabs and spaces may be included.
undefine variable
Remove variable from the list of environment variables.
export variable
Mark the variable variable to be exported to the remote side.
unexport variable
Mark the variable variable to not be exported unless explicitly asked for by the remote
side.
list List the current set of environment variables. Those marked with a * will be sent auto‐
matically, other variables will only be sent if explicitly requested.
? Prints out help information for the environ command.
LINEMODE option, then enter ``character at a time'' mode.
line Enable the TELNET LINEMODE option, or, if the remote side does not understand the
LINEMODE option, then attempt to enter ``old-line-by-line'' mode.
isig (-isig) Attempt to enable (disable) the TRAPSIG mode of the LINEMODE option. This requires
that the LINEMODE option be enabled.
edit (-edit) Attempt to enable (disable) the EDIT mode of the LINEMODE option. This requires that
the LINEMODE option be enabled.
softtabs (-softtabs)
Attempt to enable (disable) the SOFT_TAB mode of the LINEMODE option. This requires
that the LINEMODE option be enabled.
litecho (-litecho)
Attempt to enable (disable) the LIT_ECHO mode of the LINEMODE option. This requires
that the LINEMODE option be enabled.
? Prints out help information for the mode command.
open host [-l user] [[-] port]
Open a connection to the named host. If no port number is specified, telnet will attempt to contact
a TELNET server at the default port. The host specification may be either a host name (see hosts(5))
or an Internet address specified in the ``dot notation'' (see inet(3)). The -l option may be used to
specify the user name to be passed to the remote system via the ENVIRON option. When connecting to a
non-standard port, telnet omits any automatic initiation of TELNET options. When the port number is
preceded by a minus sign, the initial option negotiation is done. After establishing a connection,
the file .telnetrc in the user's home directory is opened. Lines beginning with a ``#'' are comment
lines. Blank lines are ignored. Lines that begin without whitespace are the start of a machine
entry. The first thing on the line is the name of the machine that is being connected to. The rest
of the line, and successive lines that begin with whitespace are assumed to be telnet commands and
are processed as if they had been typed in manually to the telnet command prompt.
quit Close any open TELNET session and exit telnet. An end-of-file (in command mode) will also close a
session and exit.
send arguments
Sends one or more special character sequences to the remote host. The following are the arguments
which may be specified (more than one argument may be specified at a time):
abort Sends the TELNET ABORT (Abort processes) sequence.
ao Sends the TELNET AO (Abort Output) sequence, which should cause the remote system to flush
all output from the remote system to the user's terminal.
ayt Sends the TELNET AYT (Are You There) sequence, to which the remote system may or may not
choose to respond.
brk Sends the TELNET BRK (Break) sequence, which may have significance to the remote system.
ec Sends the TELNET EC (Erase Character) sequence, which should cause the remote system to erase
the last character entered.
getstatus
If the remote side supports the TELNET STATUS command, getstatus will send the subnegotiation
to request that the server send its current option status.
ip Sends the TELNET IP (Interrupt Process) sequence, which should cause the remote system to
abort the currently running process.
nop Sends the TELNET NOP (No OPeration) sequence.
susp Sends the TELNET SUSP (SUSPend process) sequence.
synch Sends the TELNET SYNCH sequence. This sequence causes the remote system to discard all pre‐
viously typed (but not yet read) input. This sequence is sent as TCP urgent data (and may
not work if the remote system is a 4.2BSD system -- if it doesn't work, a lower case ``r''
may be echoed on the terminal).
do cmd Sends the TELNET DO cmd sequence. cmd can be either a decimal number between 0 and 255, or a
symbolic name for a specific TELNET command. cmd can also be either help or ? to print out
help information, including a list of known symbolic names.
dont cmd
Sends the TELNET DONT cmd sequence. cmd can be either a decimal number between 0 and 255, or
a symbolic name for a specific TELNET command. cmd can also be either help or ? to print out
help information, including a list of known symbolic names.
will cmd
Sends the TELNET WILL cmd sequence. cmd can be either a decimal number between 0 and 255, or
a symbolic name for a specific TELNET command. cmd can also be either help or ? to print out
help information, including a list of known symbolic names.
wont cmd
Sends the TELNET WONT cmd sequence. cmd can be either a decimal number between 0 and 255, or
a symbolic name for a specific TELNET command. cmd can also be either help or ? to print out
help information, including a list of known symbolic names.
? Prints out help information for the send command.
set argument value
unset argument value
The set command will set any one of a number of telnet variables to a specific value or to TRUE. The
special value off turns off the function associated with the variable; this is equivalent to using
the unset command. The unset command will disable or set to FALSE any of the specified functions.
The values of variables may be interrogated with the display command. The variables which may be set
or unset, but not toggled, are listed here. In addition, any of the variables for the toggle command
may be explicitly set or unset using the set and unset commands.
ayt If TELNET is in localchars mode, or LINEMODE is enabled, and the status character is typed, a
TELNET AYT sequence (see send ayt preceding) is sent to the remote host. The initial value
for the "Are You There" character is the terminal's status character.
echo This is the value (initially ``^E'') which, when in ``line by line'' mode, toggles between
doing local echoing of entered characters (for normal processing), and suppressing echoing of
entered characters (for entering, say, a password).
flushoutput
If telnet is in localchars mode (see toggle localchars below) and the flushoutput character
is typed, a TELNET AO sequence (see send ao above) is sent to the remote host. The initial
value for the flush character is taken to be the terminal's flush character.
forw1
forw2 If TELNET is operating in LINEMODE, these are the characters that, when typed, cause partial
lines to be forwarded to the remote system. The initial value for the forwarding characters
are taken from the terminal's eol and eol2 characters.
interrupt
If telnet is in localchars mode (see toggle localchars below) and the interrupt character is
typed, a TELNET IP sequence (see send ip above) is sent to the remote host. The initial
value for the interrupt character is taken to be the terminal's intr character.
kill If telnet is in localchars mode (see toggle localchars below), and if telnet is operating in
``character at a time'' mode, then when this character is typed, a TELNET EL sequence (see
send el above) is sent to the remote system. The initial value for the kill character is
taken to be the terminal's kill character.
lnext If telnet is operating in LINEMODE or ``old line by line'' mode, then this character is taken
to be the terminal's lnext character. The initial value for the lnext character is taken to
be the terminal's lnext character.
quit If telnet is in localchars mode (see toggle localchars below) and the quit character is
typed, a TELNET BRK sequence (see send brk above) is sent to the remote host. The initial
value for the quit character is taken to be the terminal's quit character.
reprint
If telnet is operating in LINEMODE or old line by line'' mode, then this character is taken
to be the terminal's reprint character. The initial value for the reprint character is taken
to be the terminal's reprint character.
rlogin This is the rlogin escape character. If set, the normal TELNET escape character is ignored
unless it is preceded by this character at the beginning of a line. This character, at the
beginning of a line, followed by a "." closes the connection; when followed by a ^Z it sus‐
pends the telnet command. The initial state is to disable the rlogin escape character.
start If the TELNET TOGGLE-FLOW-CONTROL option has been enabled, then this character is taken to be
the terminal's start character. The initial value for the start character is taken to be the
terminal's start character.
stop If the TELNET TOGGLE-FLOW-CONTROL option has been enabled, then this character is taken to be
the terminal's stop character. The initial value for the stop character is taken to be the
terminal's stop character.
susp If telnet is in localchars mode, or LINEMODE is enabled, and the suspend character is typed,
a TELNET SUSP sequence (see send susp above) is sent to the remote host. The initial value
for the suspend character is taken to be the terminal's suspend character.
tracefile
This is the file to which the output, caused by netdata or option tracing being TRUE, will be
S/Key system.
slc state The slc command (Set Local Characters) is used to set or change the state of the special characters
when the TELNET LINEMODE option has been enabled. Special characters are characters that get mapped
to TELNET commands sequences (like ip or quit) or line editing characters (like erase and kill). By
default, the local special characters are exported.
check Verify the current settings for the current special characters. The remote side is
requested to send all the current special character settings, and if there are any dis‐
crepancies with the local side, the local side will switch to the remote value.
export Switch to the local defaults for the special characters. The local default characters
are those of the local terminal at the time when telnet was started.
import Switch to the remote defaults for the special characters. The remote default characters
are those of the remote system at the time when the TELNET connection was established.
? Prints out help information for the slc command.
status Show the current status of telnet. This includes the peer one is connected to, as well as the cur‐
rent mode.
toggle arguments [...]
Toggle (between TRUE and FALSE) various flags that control how telnet responds to events. These
flags may be set explicitly to TRUE or FALSE using the set and unset commands listed above. More
than one argument may be specified. The state of these flags may be interrogated with the display
command. Valid arguments are:
authdebug Turns on debugging information for the authentication code.
autoflush If autoflush and localchars are both TRUE, then when the ao or quit characters are rec‐
ognized (and transformed into TELNET sequences; see set above for details), telnet
refuses to display any data on the user's terminal until the remote system acknowledges
(via a TELNET TIMING MARK option) that it has processed those TELNET sequences. The
initial value for this toggle is TRUE if the terminal user had not done an "stty
noflsh", otherwise FALSE (see stty(1)).
autodecrypt When the TELNET ENCRYPT option is negotiated, by default the actual encryption (decryp‐
tion) of the data stream does not start automatically. The autoencrypt (autodecrypt)
command states that encryption of the output (input) stream should be enabled as soon
as possible.
autologin If the remote side supports the TELNET AUTHENTICATION option TELNET attempts to use it
to perform automatic authentication. If the AUTHENTICATION option is not supported,
the user's login name are propagated through the TELNET ENVIRON option. This command
is the same as specifying a option on the open command.
autosynch If autosynch and localchars are both TRUE, then when either the intr or quit character
is typed (see set above for descriptions of the intr and quit characters), the result‐
ing TELNET sequence sent is followed by the TELNET SYNCH sequence. This procedure
should cause the remote system to begin throwing away all previously typed input until
both of the TELNET sequences have been read and acted upon. The initial value of this
toggle is FALSE.
line feed. This mode does not affect those characters typed by the user, only those
received from the remote host. This mode is not very useful unless the remote host
only sends carriage return, but never line feeds. The initial value for this toggle is
FALSE.
debug Toggles socket level debugging (useful only to the superuser). The initial value for
this toggle is FALSE.
encdebug Turns on debugging information for the encryption code.
localchars If this is TRUE, then the flush, interrupt, quit, erase, and kill characters (see set
above) are recognized locally, and transformed into (hopefully) appropriate TELNET con‐
trol sequences (respectively ao, ip, brk, ec, and el; see send above). The initial
value for this toggle is TRUE in ``old line by line'' mode, and FALSE in ``character at
a time'' mode. When the LINEMODE option is enabled, the value of localchars is
ignored, and assumed to always be TRUE. If LINEMODE has ever been enabled, then quit
is sent as abort, and eof and suspend are sent as eof and susp (see send above).
netdata Toggles the display of all network data (in hexadecimal format). The initial value for
this toggle is FALSE.
options Toggles the display of some internal telnet protocol processing (having to do with
TELNET options). The initial value for this toggle is FALSE.
prettydump When the netdata toggle is enabled, if prettydump is enabled the output from the
netdata command will be formatted in a more user readable format. Spaces are put
between each character in the output, and the beginning of any TELNET escape sequence
is preceded by a '*' to aid in locating them.
skiprc When the skiprc toggle is TRUE, TELNET skips the reading of the .telnetrc file in the
user's home directory when connections are opened. The initial value for this toggle
is FALSE.
termdata Toggles the display of all terminal data (in hexadecimal format). The initial value
for this toggle is FALSE.
verbose_encrypt
When the verbose_encrypt toggle is TRUE, telnet prints out a message each time encryp‐
tion is enabled or disabled. The initial value for this toggle is FALSE.
? Displays the legal toggle commands.
z Suspend telnet. This command only works when the user is using the csh(1).
! [command]
Execute a single command in a subshell on the local system. If command is omitted, then an interac‐
tive subshell is invoked.
? [command]
Get help. With no arguments, telnet prints a help summary. If a command is specified, telnet will
print the help information for just that command.
ENVIRONMENT
telnet uses at least the HOME, SHELL, DISPLAY, and TERM environment variables. Other environment variables may
system) when it is the first character on a line.
Source routing is not supported yet for IPv6.
BSD February 3, 1994 BSD
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